Shapado looks good, and its Rails based. However it has a dependency on MongoDB , which makes it less suitable for widespread deployment, as well as being licensed under the GNU Affero General Public license, which is something to be wary of.
Haven't used it, but there's also OSQA, if you can run your own server. OSQA is Django-based, whereas Shapado has a hosted offering and is RubyOnRails-based.
They both appear to be close copies of StackExchange from a cursory look.